The present invention is directed to a MOSFET switching circuit and more specifically, to a MOSFET switching circuit used for sampling analog signals.
There are several characteristics of the MOSFET type device that make it attractive as a circuit element for analog switching applications. Aside from fabrication properties, the device as a circuit element exhibits essentially infinite input impedance, zero output offset voltage very high source-to-drain resistance in the off state (i.e. good isolation) and relatively low source-to-drain resistance in the on state. The latter property is true for operation in the triode region but not for saturation operation. The source-to-drain resistance in the on state, R.sub.on, is typically 30 to 300 ohms for an N-channel type MOSFET. Unfortunately, R.sub.ON is a variable function of temperature, T, and gate-to-source voltage, V.sub.GS. An increase in device temperature will cause an increase in device resistance. Large increases in temperature can cause changes in R.sub.ON by as much as two times the lowest value of R.sub.ON. The dominant factor influencing the temperature dependence of the device is the loss carrier mobility in the inversion layer as temperature increases. R.sub.ON also varies inversely with gate-to-source voltage. Analog voltage variations across the gate and source electrodes can cause noticeable changes in R.sub.ON. Quantitatively voltage variations can cause corresponding changes in resistance by as much as three times the lowest value of R.sub.ON.
It is therefore, a broad object of the present invention to provide an improved MOSFET switching circuit having a constant R.sub.ON.
It is also an object of the present invention to provide an analog MOSFET switching circuit having a compensation circuit which varies the gate-to-source voltage in such a way as to stabilize the value of R.sub.ON despite changes in ambient temperature or despite analog signal variations.